Recently, several research groups have reported the growth of germanene, a new member of the graphene family. Germanene is in many aspects very similar to graphene, but in contrast to the planar graphene lattice, the germanene honeycomb lattice is buckled and composed of two vertically displaced sub-lattices. Density functional theory calculations have revealed that free-standing germanene is a 2D Dirac fermion system, i.e. the electrons behave as massless relativistic particles that are described by the Dirac equation, which is the relativistic variant of the Schrodinger equation. Germanene is a very appealing 2D material. The spin-orbit gap in germanene (~24 meV) is much larger than in graphene (<0.05 meV), which makes germanene the ideal candidate to exhibit the quantum spin Hall effect at experimentally accessible temperatures. Additionally, the germanene lattice offers the possibility to open a band gap via for instance an externally applied electrical field, adsorption of foreign atoms or coupling with a substrate. This opening of the band gap paves the way to the realization of germanene based field-effect devices. In this topical review we will (1) address the various methods to synthesize germanene (2) provide a brief overview of the key results that have been obtained by density functional theory calculations and (3) discuss the potential of germanene for future applications as well for fundamentally oriented studies.

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Germanene: the germanium analogue of graphene.

Rhenium disulfide (ReS2) is a two-dimensional (2D) group VII transition metal dichalcogenide (TMD). It is attributed with structural and vibrational anisotropy, layer-independent electrical and optical properties, and metal-free magnetism properties. These properties are unusual compared with more widely used group VI-TMDs, e.g., MoS2, MoSe2, WS2 and WSe2. Consequently, it has attracted significant interest in recent years and is now being used for a variety of applications including solid state electronics, catalysis, and, energy harvesting and energy storage. It is anticipated that ReS2 has the potential to be equally used in parallel with isotropic TMDs from group VI for all known applications and beyond. Therefore, a review on ReS2 is very timely. In this first review on ReS2, we critically analyze the available synthesis procedures and their pros/cons, atomic structure and lattice symmetry, crystal structure, and growth mechanisms with an insight into the orientation and architecture of domain and grain boundaries, decoupling of structural and vibrational properties, anisotropic electrical, optical, and magnetic properties impacted by crystal imperfections, doping and adatoms adsorptions, and contemporary applications in different areas.

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Advent of 2D Rhenium Disulfide (ReS2): Fundamentals to Applications

The great success of graphene has encouraged the fast development of other two-dimensional (2D) nanosheets, which have attracted extensive attention in different scientific fields encompassing field effect transistors, lithium-ion batteries, and catalysis. With atomic-scale thickness, almost all of the atoms are exposed on the surface, providing an extremely high specific surface area, in conjunction with special physical, chemical, and electronic properties, owing to the quantum confinement effects, which enable their surface phase to be as important as the bulk counterparts. In this review, we have summarized and discussed the recent advancements of 2D nanomaterials beyond graphene, with an emphasis on their basic fundamentals, preparation strategies, and applications. We believe that this review supplies critical insights for exploring and understanding 2D nanomaterials and puts forward the challenges and opportunities for further developments, such as more precise morphology control, foreign atom doping and surface modification technologies, atomic-scale characterization, and finding wide applications in many different fields.

Elemental two-dimensional nanosheets beyond graphene

The field of organic electronics has been heavily impacted by the discovery and development of π-conjugated conducting polymers. These polymers show great potential for integration into future optical and electronic devices due to their capacity to transition between semiconducting and conducting states as well as the ability to alter mechanical properties by controlled doping, chemical modification, and stacking or creating composites with other materials. Among π-conjugated polymers, polythiophene and its derivatives has been one of the most extensively studied and is widely investigated computationally and experimentally for use in electronic devices such as light-emitting diodes, water purification devices, hydrogen storage, and biosensors. Various theoretical modeling studies of polythiophene ranging from an oligothiophene approach to infinite chain lengths (periodic boundary conditions) have been undertaken to study a variety of electronic and structural properties of these polymers. In this review,...

Polythiophene: From Fundamental Perspectives to Applications

Using first-principles calculations, we investigate the adsorption characteristics of alkali, alkaline-earth, nonmetallic, transition, and noble metal adatoms on phosphorene. The adsorption-induced tunable electronic structures are comparatively studied on two representative structures of phosphorene, i.e., black and blue phosphorus (P) nanosheets. Both black and blue P sheets exhibit good adsorption capability to foreign atoms, on which the binding energies of adatoms are stronger than on the BN, SiC, MoS2, or graphene sheets. On the black P sheet, most adatoms prefer to adsorb on the hollow site, whereas for the blue P sheet, the favored adsorption sites are element-dependent. The majority of alkali, alkaline-earth, and transition-metal adatoms prefer the valley site, noble metal adatoms the hollow site, and nonmetallic adatoms the bridge and top sites instead. The semiconducting behaviors of phosphorene are modified by adatoms, which can cause p-type and n-type doping or induce midgap states into the P...

Structural, Electronic, and Magnetic Properties of Adatom Adsorptions on Black and Blue Phosphorene: A First-Principles Study

The structural, electronic, and magnetic properties of 3$d$ transition metal (TM) atoms (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) adsorbed germanene are addressed using density functional theory. Based on the adsorption energy, TM atoms prefer to occupy at the hollow site for all the cases. The obtained values of the total magnetic moment vary from 0.97 $\mu_B$ to 4.95 $\mu_B$ in case of Sc to Mn-adsorption, respectively. A gap of 74 meV with a strongly enhanced splitting of 67 meV is obtained in case of Sc-adsorption, whereas metallic states are obtained in case of Ti, Cr, Mn, Fe, and Co. Non-magnetic states are realized for Ni, Cu, and Zn-adsorption. Moreover, semiconducting nature is obtained for non-magnetic cases with a gap of 26 to 28 meV. Importantly, it is found that V-adsorbed germanene can host the quantum anomalous Hall effect. The obtained results demonstrate that TM atoms and nearest neighbour Ge atoms are ferro-magnetically ordered in the cases of V, Mn, Fe, Co, Ni, Cu, and Zn, while anti-ferromagnetic ordering is obtained for Sc, Ti, and Cr. In addition, the effects of the coverage of all TM atoms on the electronic structure and the ferro-magnetic and anti-ferro-magnetic coupling in case of Mn are examined. The results could help to understand the effect of TM atoms in a new class of two-dimensional materials beyond graphene and silicene.

T. P. Kaloni

Papers

Tuning the Structural, Electronic, and Magnetic Properties of Germanene by the Adsorption of 3$d$ Transition Metal Atoms